1. Field of the Invention
The present invention relates to an active damping apparatus of fluid-filled type, adapted to provide dynamic vibration damping action by means of actively controlling pressure fluctuations in a pressure receiving chamber having non-compressible fluid sealed therein, in cycle corresponding to the frequency of vibration to be damped. More particularly, the invention relates to a fluid-filled active damping apparatus of novel construction employing an electromagnetic actuator in order to control pressure fluctuations in the pressure-receiving chamber.
2. Description of the Related Art
A variety of types of damping apparatus known in the art for vibration in a component to be damped, such as a vehicle body, include an apparatus of vibration damping type that utilize damping action of a shock absorber, rubber elastic body or the like, and an apparatus of vibration isolation type that utilize the spring action of a coil spring, rubber elastic body or the like. However, all of these damping apparatus utilize passive vibration damping action. Thus, if the frequency or other characteristic of vibration to be damped changes, or if a higher level of vibration damping action is required, the known passive damping apparatus are difficult to adequately achieve the desired vibration damping effect.
In recent years, there have been developed and tested active damping apparatus that utilize an actuator to generate oscillation force in a cycle corresponding to the frequency of the vibration being damped, so as to actively reduce vibration. In this kind of active damping apparatus, a high degree of controllability is required in relation to the frequency and phase of the oscillation force generated by the actuator which generates the oscillation force. Thus, as the actuator there may be favorably employed, for example, an electromagnetic actuator comprising a coil and an output member such as an armature that receives driving force when current is passed through the coil, wherein the output exerted on the output member is controlled by means of the action of electromagnetic force or magnetic force by means of controlling current flow to the coil.
More specifically, an active damping apparatus of the above type, as taught for example in JP-A-2001-1765, includes a mounting assembly having a rubber elastic body elastically connecting a first mounting member and a second mounting member, a pressure receiving chamber partially defined by the rubber elastic body with a non-compressible fluid sealed therein to which an input vibration is applied, and adapted to receive, and an oscillation plate elastically supported in displaceable fashion, and partially defining the pressure receiving chamber. An electromagnetic actuator is disposed on a first side of the oscillation plate remote from the pressure-receiving chamber, and a coil which constitutes part of the actuator is supported fixedly by the second mounting member. To the oscillating plate, there is fixed an output member on which driving force is exerted by energizing the coil. By means of this arrangement, driving force is exerted on the oscillation plate by energizing the coil, causing excitation displacement of the oscillation plate. With this arrangement, the pressure in the pressure-receiving chamber can be actively controlled to produce vibration-damping action.
In an electromagnetic actuator employed in a fluid-filled active damping apparatus of this kind, in order to effectively achieve the desired vibration damping action, it is necessary that the oscillation force created by energizing the coil be produced consistently at a desired magnitude. By producing oscillation force of magnitude corresponding to input vibration in order to reduce dynamic spring constant on the basis of pressure fluctuations of the pressure receiving chamber being absorbed through dynamic excitation displacement of the oscillation plate for example, it is possible to appreciably improved control of vibration damping ability.
Thus, in an electromagnetic actuator of this kind it is necessary that the relative positional relationship of the output member to the coil be established with a high degree of accuracy. This is because while the magnitude of magnetic force or electromagnetic force produced in the output portion is intimately related to the magnitude of flux density at the location where the output member is disposed, flux density magnitude varies appreciably with relative position to (distance apart from) the coil which is the source of the magnetic field.
In the fluid-filled active damping apparatus of the construction described above, the oscillation plate is elastically supported by and positioned relative to the second mounting member via a support rubber elastic body of circular plate shape or annular plate shape, whereby displacement of the oscillation plate in the axial direction can be permitted on the basis of elastic deformation of the support rubber elastic body. Thus, there is a tendency for the support rubber elastic body to suffer from its fatigue relatively early, due to the elastic deformation produced repeatedly in the support rubber elastic body during displacement of the oscillation plate. When the support rubber elastic body has fatigued, created are a change in the support location of the oscillation plate, and accordingly in the relative position of the coil to the output member in the electromagnetic actuator, resulting in the problem of difficulty in consistently achieving the desired drive force and damping effect. Additionally, since excitation energy is consumed by deformation of the support rubber elastic body, there is the problem of lower drive efficiency of the actuator.
To meet this problem, as taught for example in JP-A-6-330980, it has been contemplated to form a through-hole in part of the wall of the pressure receiving chamber, and to dispose the oscillation plate displaceably accommodated within the through-hole, without needing the support rubber body connecting between the oscillation plate and the through hole. With this arrangement, the oscillation plate is supported without interposition of the support rubber elastic body mentioned previously, thus avoiding adverse effects of fatigue of the support rubber elastic body on driving of the oscillation plate.
However, in the fluid-filled active damping apparatus taught in JP-A-6-330980, there is a need for the gap between the opposed outer peripheral face of the oscillation plate and inner peripheral face of the through-hole to be made smaller in order to prevent leakage of pressure in the pressure receiving chamber to the outside through the through-hole while assuring good axial displacement of the oscillation plate within the through-hole. Thus, it is necessary to establish both the through-hole inside diameter dimension and the oscillation plate outside diameter dimension with a very high degree of precision, which presents the problem of difficulty in manufacture and maintenance.
Also, when the oscillation plate is induced to undergo drive displacement, at least part of the oscillation plate is likely to come into contact with the through-hole, due to the small size of the gap between the oscillation plate and the through-hole. Additionally, the oscillation plate is mounted on the output shaft of the electromagnetic actuator and is fitted into the through-hole during mounting of the electromagnetic actuator. Thus, any error (deviation) in mounting position of the electromagnetic actuator on the mounting assembly will easily result in the center axes of the oscillation plate and the through-hole being misaligned. Due to this misalignment of their center axes, the oscillation plate will tend to experience interference with the inner peripheral face of the through-hole, even where each of the components has high dimensional accuracy. Resultant contact of the oscillation plate and the through-hole poses the risk of producing a hammering noise, or of difficulty in achieving the desired oscillation force due to the difficulty of efficiently realizing drive displacement of the oscillation plate. Thus, an inherent problem is difficulty in consistently achieving drive of the oscillation plate, and difficulty in consistently exhibiting the intended vibration damping effect.